Now this will sound bonkers, but since upgrading to the latest ~Gnome3.8 and systemd, I was left with a black screen when logging in.

I am presented with GDM from where I log in, and then I am left with a blank screen.
Sometimes it will load the desktop AFTER 4 MINUTES!!! and other times, nothing.

Even after waiting for 4 minutes, I am taken back to the GDM login screen. !!

I removed networkmanager from start up and the blank screen problem was solved. How weird.

I do think however, my solution will not work for all.

This seems to be a somewhat "endemic" problem, as I had the exact same problem: my computer would boot, X server would be started, but GDM would only appear sometimes, and if it did, Gnome would never start after logging in. And no matter how hard I tried to look for (obvious) errors, there were none... Finally, I removed systemd, removed Gnome-3.8, then installed OpenRC and XFCE...